Oh, there were more than four most likely, don't forget the vastness of the Galactic Empire.
Just between the start of production on the Imperator
-Class to the rebirth of the Emperor 6 years after RotJ, over 20,000 ISD mk-I was built. Don't know how many of the mk-II was built though. And that's just one class of star destroyer. You still have the Victory
-Class mk-I and mk-II, Imperator
-Class mk-II, Allegiance
-Class, and the Eclipse
even though this was over the course of 6 years past RotJ, there is bound to have been built more than 4 SSD's of the Executor
-Class before the second Death Star blew up.
Known named Executor-Class
ships before the Battle of Endor:
That's 7 just there. NJO haven't done it's research I guess.
Total known named ships before and after RotJ is:
I don't think that is the complete list though, because there are several pages long list of names on different star destroyer classes I would have to go thru in order to filter them out.
I haven't gone over it in awhile.
Check out this site for all, or atleast most of the names used on different star destroyer classes in Star Wars: The Complete List of Star Destroyer Names